Yes, this is something that I find very interesting - around which part of evolution would whales have began to lose their hair? As I understand it, the general theory is that whales such as Ambulocethus still had hair, possibly similar to that of some seals or otters.

But how do you get from there to something like Dorudon? Which I believe have been considered to have skin/coating more similar to modern whales, i.e almost no hair whatsoever.

What would the stages in-between have been like? And are there any other semi-aquatic or aquatic mammals that can be used as a comparisson? For instance, we humans retain some hair on our head (for other reasons), but what parts of the whales would have started thinning out hair-wise, first? The belly perhaps?

Would definitely be interesting to find some more info on this, since I think the visual of these prehistoric semi-bald whales could have been quite interesting indeed. =)

see below - which doesn't provide an estimate when the switch ocured but does inform the discussion.